This application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 10/795,023, now U.S. Pat. No. 7,088,181 filed Mar. 5, 2004. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application.
The following disclosure relates to electrical circuits.
Differential amplifiers are useful circuits for a variety of applications. For example, a differential amplifier can be implemented as a wideband transimpedance amplifier within a high speed (e.g., a 10 GHz) communication system. Differential amplifiers typically include a common mode feedback (CMFB) circuit. The CMFB circuit operates to stabilize an associated differential amplifier—i.e., set a direct voltage (DC) operating point at the output of the differential amplifier.
FIG. 1 illustrates a conventional CMFB circuit 100 for a differential amplifier 102. CMFB circuit 100 includes an error amplifier A1 that compares a common mode voltage (as measured at node 104) to a reference voltage VREF. Error amplifier A1 adjusts the common mode voltage by supplying a feedback voltage 106 to the gates of resistive current mirror transistors M14, M15, M16, M17. For example, as the common mode voltage increases, error amplifier A1 increases feedback voltage 106 being supplied to the gates of resistive current mirror transistors M14, M15, M16, M17. Less current, therefore, flows through resistive current mirror transistors M14, M15, M16, M17. As a result, more current flows through resistors R1, R2 and the output voltage of differential amplifier 102 (and the common mode voltage) decreases. The feedback path of error amplifier A1 passes from the output of error amplifier A1 to the gates of resistive current mirror transistors M14, M15, M16, M17, through the outputs of wideband amplifier 102, and back to a positive input of error amplifier A1.
A switch 108 can be used to turn off differential amplifier 102 during a power down mode, for example, to save power consumption. Switch 108 can be implemented with a CMOS transmission gate. As noted above, the feedback path of CMFB circuit 100 passes through circuitry of differential amplifier 102. Thus when differential amplifier 102 is powered off (e.g., by opening switch 108), CMFB circuit 100 is also typically powered off. The direct voltage (DC) operating point at the output of the differential amplifier is not retained as the feedback of CMFB circuit 100 path ceases to exist (even if error amplifier A1 remains on).
A typical problem associated with switching a differential amplifier on and off is the differential amplifier may exhibit a relatively long settling time due to the feedback path of a CMFB circuit. The long settling time is due to error amplifier A1 having to charge a relatively large feedback loop capacitance CL. The long settling time can degrade performance of the differential amplifier, particularly if a loop gain of the CMFB circuit 100 is relatively large.